Essential Linux System Administration Concepts

1. Lecture 13 Linux System Administration

2. Booting • PROM (BIOS) — perform basic self-test (POST) and access parameters from nvram • OS Loader — locate and run kernel on disk • Located in the MBR (first sector of boot device) • May call secondary loader on some partition • LILO, GRUB • Kernel — initializes devices, mounts root filesystem, starts first user process (init)

3. init • init — reads /etc/inittab to determine what to start according to the run-level(initdefault) run-level

4. Boot Scripts • /etc/init.d contains scripts for every managed service, e.g. /etc/init.d/sshd {start|stop} • Links to these boot scripts are created in the sequencing directories/etc/rc[0-6].d • Links started with S are called with start • Links started with K are called with stop

5. Boot Scripts (cont.) • Numbers in link determine the order the script are run, e.g. • S55sshd runs before S80sendmail but after S08iptables • Maintain runlevel information for system services by manipulating files in /etc/rc[0-6].d or use chkconfig

6. Internet Services Daemon • xinetd — listens to service ports and starts server when a request arrives • No need to start all the daemons at boot time • “Super-server” • Services are configured in /etc/xinetd.conf or in individual files under /etc/xinetd.d

7. Shutting Down • shutdown brings the system down safely : /sbin/shutdown -t 600 -r “… be right back” • Processes are sent SIGTERM and then SIGKILL • halt same as shutdown -h • reboot same as shutdown -r • poweroff turns off the power after halting (same as halt -p)

8. User Account Management • Local user info stored in /etc/passwd • To create a new local user : • Add new entry to /etc/passwd and /etc/shadow (and /etc/group is necessary) • Create home directory for the new user with some default startup files • Do these manually or use useradd : useradd -c “Bill Gates” -u 1001 -g msoft -d /home/billg -m -k /etc/skel -s /bin/bash billg

9. User Acct. Management (cont.) • To delete an account : userdel -r billg • To create a group : groupadd -g 550 web • To delete a group : groupdel web

10. /etc/passwd • Format of a passwd entry: username:password:uid:gid:gecos:homedir:shell root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin nobody:x:99:99:Nobody:/:/sbin/nologin billg:x:1001:501:Bill Gates:/home/billg:/bin/bash

11. /etc/shadow • Format of a shadow entry: username:password:lstchg:min:max:warn:inact:exp: root:j3dghRBqe$2fjvGJ8js:12650:0:99999:7::: bin:*:12650:0:99999:7::: … • * does not match any password • !! account is locked • The shadow file should only be readable by root

12. Groups • Format of a group entry in /etc/group groupname:password:gid:user_list root::0:root bin::1:root,bin,daemon senate::990:chuck,hillary • Group passwords can be stored in /etc/gshadow • If you belong to more than 1 groups, you can change your group with : newgrp [ group ]

13. Become Another User • su - run shell as another user • Need password of the user you are su’ing to • No username specified means root • sudo - execute command as another user • Authenticate with your own password • Run command as root by default • sudo privileges are defined in /etc/sudoers

14. Installation • Install from CD/DVDs interactively • Network automated installation • Kickstart (Red Hat) • Jumpstart (Solaris) • Packages and machine configuration files located on install server • Install a machine with a single command linux ks=nfs:server:/path(RH Linux) boot net - install (Solaris)

15. Linux Distros

16. Disk Partition • A partition is a logical section of a disk, normally with its own filesystem • The partition table contains the partition information (starting block, size, type) • A disk can be partitioned during OS installation or (for non-system disks) afterwards using fdisk or parted

17. A Partition Table (parted) print Disk geometry for /dev/hda: 0.000-38146.972 megabytes Disk label type: msdos Minor Start End Type Filesystem Flags 1 0.031 25603.593 primary ntfs boot 2 25603.594 25705.568 primary ext3 3 25705.569 26733.164 primary linux-swap 4 26733.164 38146.530 extended lba 5 26733.195 38146.530 logical ext3

18. Filesystems • Different filesystem types organize files and directories in different ways • Ext3 — most common filesystem on Linux • Ext3 is a journaling filesystem • Sequence of changes to filesystem treated as single transaction • After unclean system shutdown • Replay journal to make filesystem consistent • No need to fsck

19. Mounting Filesystems /etc/fstab: LABEL=/ / ext3 defaults 1 1 LABEL=/boot /boot ext3 defaults 1 2 none /proc proc defaults 0 0 /dev/sda2 swap swap defaults 0 0 • mount -a causes all fs in fstab to be mounted • To manually mount a filesystem not in fstab mount -t ext3 -o ro,acl /dev/sda5 /a • To check filesystem usage, use df, e.g. df /usr

20. Access Control Lists (ACL) • Traditionally, file permissions can only be set for user, group, and everyone else • Simple but limited • Different perms cannot be used for different users • ACL provides finer access control • Filesystems need to be mounted with the acl option

21. Setting ACL • To give Prof. Korn rw access to your file that has permission 600: setfacl -m u:kornj:rw somefile • To remove all permission for Prof. Korn: setfacl -x u:kornj somefile • To list the ACL for a file/directory: getfacl somefile

22. Quota • Prevent one user from using up the whole disk • Disk quota can be configured for individual users as well as groups • To enable quota on a filesystem, mount with usrquota and/or grpquota options

23. Setting Disk Quota • To list quota for user or group: quota user orquota -g group Disk quotas for user foo (uid: 501): Filesystem blocks soft hard inodes soft hard /dev/sdb2 223652 512000 600000 23456 0 0 • User can exceed soft limit for a grace period • To configure quota for user: edquota user • To configure quota for group: edquota -g group

24. Swap • Swap space — area on disk for transferring pages to/from physical memory (RAM) • When RAM is (almost) full, RAM pages are saved to swap by the page daemon • Can be a dedicated partition or a swap file • Usually twice the size of RAM • e.g. 2048 MB swap for 1024 MB RAM

25. RAID • Redundant Array of Independent Disks • Combine multiple smaller physical disks into one big logical disk: OS sees one big drive • Improve I/O performance and provide redundancy • Most common RAID levels • Linear : concatenation • RAID 0 : striping - no redundancy • RAID 1 : mirroring • RAID 5 : striping with distributed-parity (XOR) • RAID 6 : P + Q redundancy - up to 2 disk failure

26. RAID Level 5 Disk 1 Disk 2 Disk 3 Disk 4 Disk 5 0 1 2 3 P 5 6 7 P 4 10 11 P 8 9 15 P 12 13 14 P 16 17 18 19 20 21 22 23 P Left-symmetric

27. Hardware vs. Software RAID • Hardware RAID • RAID controller handles everything • Host sees one big drive • Software RAID • Kernel handles all RAID issues (MD driver) • Cheaper but lower performance • See md(4), mdadm(8)

28. Network Configuration • Ethernet devices are named eth0, eth1, etc. • To statically configure a network interface: • IP address (128.122.20.123) • Netmask (defines subnet) (255.255.255.0) • Router (gateway) address (128.122.20.1) • ifconfig is used at boot time to configure network interfaces • List configuration if no argument is given

29. DHCP • Dynamic Host Configuration Protocol • Dynamically allocate IP addresses to clients • Addresses are leased for a certain period • Some older clients use BOOTP

30. Network File System (NFS) • Developed by Sun Microsystems • Allowed remote filesystems to be mounted locally • e.g. home directory mounted on machines • To mount a filesystem from a NFS server mount -t nfs -o nosuid,intr serv1:/export/local /usr/local

31. NFS (cont.) NFS client NFS server (serv1) / / usr export via NFS local local mount point for serv1:/export/local bin lib share

32. Naming and Directory Services • Original UNIX naming system stores info in /etc • Does not scale well for large network • Network naming services • Information stored centrally (client-server model) • Usernames, passwords, hostnames/IP addr, etc. • Binds names to objects • Resolves names to objects • e.g. www.cs.nyu.edu is 128.122.80.245 • DNS, NIS, LDAP

33. Domain Name System • Distributed, replicated service for translating hostnames to IP addresses • Namespace divided into hierarchy of domains • Each DNS domain supported by 2 or more name servers

34. DNS Namespace root • gov com org edu nyu usc

35. DNS Client • The resolver (e.g. gethostbyname()) on the client queries the name server • DNS servers in /etc/resolv.conf, e.g. nameserver 128.122.128.2 • Query DNS server interactively with nslookup or dig

36. Network Information Service • Developed by Sun Microsystems - originally Yellow Pages (yp) • Stores network, hostnames-addresses, users, and network services info in NIS maps • e.g. passwd.byname, passwd.byuid, hosts.byname, ethers.byaddr, netgroup, etc. • Client-server model • Servers are replicated (master/slave) • NIS+ — similar to NIS, but more features and more secure

37. LDAP • Lightweight Directory Access Protocol • Specialized database optimized for reading and searching • What can be stored in LDAP? • Passwords, phone numbers, date-of-birth, jpeg photos, • Client-server model (again) • LDAP directory service is global • OpenLDAP is an open source implementation

38. LDAP Information Model • A LDAP entry is a collection of attributes with a unique Distinguished Name (DN) uid=jane,ou=People,dc=cims,dc=nyu,dc=edu • Each attribute has a type and one or more values telephoneNumber: 212-995-1234 • The values of the objectClass attributes decide what attributes are required/allowed objectClass: posixAccount • objectClasses are defined in schema

39. Directory Information Tree • Entries are arranged in a hierarchical structure dc=com dc=edu dc=nyu organization dc=cims ou=People ou=Hosts organizational unit uid=jane person

40. Accessing LDAP • Add, modify, and delete entries with ldapadd, ldapmodify, and ldapdelete • Search the LDAP database with ldapsearch • Bind as some DN or anonymously ldapsearch -D “cn=Directory Manager” -h ldaphost -b “dc=cims,dc=nyu,dc=edu” “uidNumber=9876” gecos • Access to information is controlled by an access control list, e.g. password hashes are not available through anonymous bind

41. Name Service Switch • Controls how a machine obtains network information, such as passwd, group, aliases, hosts, netmasks, etc. • Config file: /etc/nsswitch.conf • Sample entries: passwd: files ldap hosts: files ldap dns netmasks: files

42. Controlling Access to Services • Firewall • Packet filtering • Software vs. hardware • TCP Wrapper (IP address) • Application • Host-based (IP address, certificates) • User-based (Password) • Don’t start the daemons

43. Software Firewall - iptables • Configure tables of packet-filter rules in Linux kernel • Each table has a number of chains • Each chain consists of a list of rules • Each rule specifies what to do with a matching packet • The default table (filter) has 3 built-in chains: • INPUT incoming packets • FORWARD routed packets • OUTPUT outgoing packets

44. iptables (cont.) • Rules activated at boot time is defined in /etc/sysconfig/iptables • Sample iptables entry: -A INPUT -m state --state NEW -m tcp -p tcp -s 192.168.1.0/24 --d port 137 -j ACCEPT • Allows new TCP connections from hosts in the 192.168.1.0/24 network to port 137

45. TCP Wrapper • tcpd logs and controls incoming requests for services such as telnet, finger, rsh, etc. • inetd runs tcpd instead • tcpd logs connection and checks if connection is allowed based on hosts.allow and hosts.deny • /etc/hosts.allow: in.telnetd: .cs.nyu.edu,.cs.cuny.edu • /etc/hosts.deny: ALL: ALL

46. PAM • Pluggable Authentication Module • Centralized authentication mechanism • “Plug in” different authentication methods • Different services can have different authentication policies • Highly secure systems can require multiple passwords to authenticate

47. PAM Framework Applications ftp login ssh PAM Library libpam conf Modules pam_unix pam_ldap pam_securetty

48. PAM Stack • Modules are stacked (order is important) • Sample PAM configuration in /etc/pam.d: interface control flag module name auth required pam_nologin.so auth required pam_securetty.so auth sufficient pam_unix.so auth required pam_ldap.so

49. Date, Time, and NTP • Date sets the system date and time: date MMDDhhmm[[CC]YY][.ss] • Some applications can fail if clocks are not synchronized among machines, e.g. make • Use Network Time Protocol (NTP) • A stratum 1 server is connected to a reference clock • Stratum 2 servers synchronize with stratum1 servers • Your machine synchronized with stratum 2+ servers • Daemon: ntpd Config file: /etc/ntp.conf

50. Mail • Mail Transfer Agent (MTA) • Sendmail • Postfix • Qmail • Incoming mail are deposited into /var/mail or forwarded to another address according to the aliases (/etc/aliases) or user’s .forward